Insulators, Semiconductors, and Conductors

The electric conductivity of different materials is a consequence of their individual band structure. Metals, such as sodium, copper, aluminum, etc. have a crystalline band structure in which the upper most energy band, the valence band, is partially filled (or better yet, partially empty!). Electrons in this band can gain energy from an applied external electric potential, such as a battery, without the restriction of quantization. Insulators, on the other hand, have full valence bands, so for an electron to be promoted to its next higher available energy it has to leave this band and jump into the upper empty band. This is possible, but it takes a huge electric potential to make it happen (that's why lightning can pass through walls and kill people, but typical wiring is safe with just a thin plastic insulation over it). Semiconductors are intermediate between insulators and conductors. Their band structure is the same as an insulators' but the separating gap between their valance band and the next empty band, the conduction band, is very narrow.

Because of this narrow so called band gap, semiconductors can use the thermal energy of their lattice (collisions between the valence electrons and the oscillating lattice atoms) to promote a few of their valence electrons to the conduction band. In this way these material can conduct electricity, but because of the limited number of electrons that get promoted, they are not very good conductors. Examples of crystalline materials with narrow band gaps are silicon (Si) and germanium (Ge). In both of these materials the outermost shell is a partially filled p-shell with only two (out of a maximum of 6) electrons in it. In these crystals the two valence p-shell electrons are shared by the other neighboring atoms in the lattice.

To make a better conductor out of these semiconductors, atom impurities are introduced in the crystal formation through a process called doping. If one of the silicon atoms in the lattice is replaced by an arsenic (As) atom, which has three p-shell valence electrons, then the extra electron becomes loosely attached to its mother atom. This loose electron can take part in the electric conduction and therefore increases the conductivity of the crystal. Strangely enough, a similar process can occur if one of the silicon atoms is replaced by a gallium (Ga) atom, which has only one p-shell valence electron. In this case, the Ga impurity strongly attracts one of the p-shell electrons from one of its Si neighbors. This leads to the production of a so called "electron hole". The resulting electron vacancy (hole) can travel in the crystal just as an electron would - but in the opposite direction - and results in an increased conductivity. Doped Si crystals with extra electrons, negative carriers, are referred to as n-type; those with holes are called p-type. These impurities modify the crystals' energy band structure by introducing atomic-type energy levels between the valance and conduction bands of the crystal and thus further reduce the band gap. This is shown below: